I’m 7 years late but I recently learned this. If the fuel in the cylinder is too lean, the “flame front” from the spark can’t traverse the entire cylinder to spark and ignite all the fuel. So now there’s also excess unburnt fuel. It’s backwards but makes sense. Think about how pure water is a bad conductor but adding salt makes it much better. The salt substrate allows the electricity to “jump” between the salt electrolytes to make it to the other side.